1. Field of the Invention
The invention relates to a mechanically operated proportional flow control valves with a valve spool movement which has a non-indexed regulating range, a neutral position, which is automatically assumed if the actuating force applied to the spool is released, and at least one so-called “locked in” position at one at least of the two end limits of the regulating range in which the spool is locked in a fixed position. Hereinafter such a valve is referred to as “a valve of the type described”.
In agricultural operations a variety of attachments are used for different work. Each of these pieces of equipment makes individual demands on the vehicle-borne hydraulics and in particular on the functions of the vehicle-borne hydraulic valves and their control. Thus, there are attachments, the use of which requires hydraulic valves, which are designed to “lock-in” automatically in a certain operating position, so that the operator does not have to apply the actuating force continuously. This automatic locking may not be desired for some applications and can even lead to safety-critical operating errors. For example, with regard to the operation of a front loader some professional associations prescribe that the lifting of the front loader rocker must be interrupted if the operator is no longer operating the lever in order to prevent injury to humans and damage to equipment as a result of the lift movement continuing without conscious control by the operator. This so-called dead man circuit, can only be guaranteed with a hydraulic valve with a “locked-in” position if the “locked-in” position is disabled (or “locked-out”) for this particular application. This causes problems when it is desired to be able to use the same hydraulic valve to also control another attachment where the “locked-in” position of the hydraulic valve is required.
Controls for front loaders are known from practice, which have their own control unit only to be used for the front loader. This can be fitted in the driving cab and contains vehicle-borne controls and hydraulic valves, which on the manufacturer's side meet the requirements mentioned for “locking out” or “locking in” functions. This execution, however, has the disadvantage that, particularly for use of the front loader, control elements must be placed in the driving cab and bowden cables to the control valves on the vehicle must be routed through the cab. As a result leakages arise, which increase the sound and noise level in the cab, and high expenditure of time is required for conversion. The use of a separate command and control unit for the front loader further leads to high ancillary costs, which can be avoided by using the vehicle-borne command and control mechanics, that is to say the control valves, of the agricultural tractor.
2. Description of Related Art
Similar hydraulic control equipment for hydraulics, wherein the “lock-in” of the valve spool of a mechanically operated proportional directional control valve is switched off via hydraulic control equipment if the operating pressure is exceeded is known from German Patent DE 3801072 C1. For this purpose the control lines of the “lock-in” control are connected via a hydraulic timer to the pressure circuit. If a certain operating pressure in the circuit is exceeded the pressure is passed on via the control line and timer to a hydraulic cylinder, which releases the mechanical “lock-in” device, so that the valve control gate returns to its neutral position and therefore no pressure flows to the consumers.
From German Patent DE 3733371 A1 it is also known, instead of a hydraulically operated locking device, to use an electrohydraulic locking device, wherein the control of the locking operation is effected when a certain operating pressure is reached to actuate an electrical switch via a barometric cell, which passes on this signal via an electrical timer, here a relay, to a solenoid 2-way valve, which then operates the hydraulic cylinder of the locking device via a control line.
With both the above prior art arrangements the control of the locking device depends on the operating pressure in the system. Deliberate switching of the locking device on or off by the operator is not possible as a result. Although a device can be derived from these solutions, in which the locking can be deliberately switched on or off, this requires purely hydraulic or electrohydraulic control of a locking device and thus the use of quite costly structural parts. Moreover, major interference with the vehicle hydraulics or electrical equipment is necessary for retrofitting or for optional installation of such a locking device. Sometimes commercial valves cannot be used for installing this locking function and have to be replaced by valves which must be adapted for installing the locking device and are therefore substantially more expensive.
It is an object of the invention to provide a proportional flow control valve of the type described which overcome the above problems.